Project Summary Proper chromosome segregation during mitosis is fundamental to cell growth and organism development. Failures in this step lead to developmental diseases, immunodeficiency, and cancers. My research program aims to reveal molecular mechanisms critical for chromosome segregation and elucidate the consequences of their failures, which can be exploited to enhance the efficacy of chemotherapeutic treatments. 1) Roles of nucleosome regulators in chromosome segregation: During mitosis, chromosomes dramatically change their functions and morphology to support their segregation. Chromosomes become condensed, while promoting formation of mitotic apparatuses, such as kinetochores and spindle microtubules. Although histones are the most abundant chromatin proteins, the importance of histones in transcriptional regulation makes it challenging to dissect their direct roles in mitosis. Our lab has established a novel method to manipulate histones and evaluate their consequences using Xenopus egg extracts, which recapitulates a variety of chromatin events independently of transcription. We discovered that the chromosomal passenger complex (CPC) must interact with both nucleosomes and microtubules to support spindle assembly. As chromatin proteins regulated by the CPC, we discovered a novel nucleosome-remodeling complex composed of HELLS and CDCA7, relevant to Centromere instability and Facial anomalies (ICF) syndrome. By dissecting the roles of HELLS-CDCA7 and other nucleosome regulators, we will investigate the poorly understood mechanisms and functional significance of mitosis-specific control of nucleosome dynamics and functions. 2) Mechanisms that maintain human centromere integrity: Human centromeres, where kinetochores assemble to capture microtubules, are composed of a long array of repetitive element, ?-satellite DNA. We have shown that ?-satellite repetitive arrangement becomes unstable in cancer cells and during cellular senescence, and that centromere-binding proteins are important for preserving this arrangement. We will dissect mechanisms that maintain integrity of centromere-associated repetitive elements. 3) Recognition of mitotic failures by the innate immune system: We hypothesize that nucleosomes act as a hallmark that distinguish between genomic DNAs and foreign or aberrant DNAs. Consistent with this idea, we demonstrated that the nucleosome competitively binds and inhibits DNA-induced stimulation of the cytoplasmic DNA sensor, cGAS, a component of the innate immune system. We showed that during extended mitosis, cGAS is slowly activated and induce cell death. Cells that do not express cGAS are less prone to die during mitotic arrest induced by taxol, which is frequently used in cancer chemotherapy. Thus, cGAS expression level could be a predictor of the efficacy of taxol, and cGAS mediated-immunity may affect cytotoxic effect of taxol on tumors. We will investigate mechanisms behind cGAS-induced cell death and its relevance to clinical treatment with taxol.